Quaternary carbon stereogenic centers through copper-catalyzed enantioselective allylic substitutions with readily accessible aryl- or heteroaryllithium reagents and aluminum chlorides

Article abstract of DOI:10.1002/anie.201005124

The case of the notorious aryls is solved: The first efficient catalytic and enantioselective method for allylic substitutions that furnish quaternary carbon stereogenic centers by additions of aryl- or heteroarylmetals is reported (see scheme). Highly site- and enantioselective processes begin with readily available organolithium reagents.

Full text of DOI:10.1002/anie.201005124

Communications
DOI: 10.1002/anie.201005124
Enantioselective Catalysis
Quaternary Carbon Stereogenic Centers through Copper-Catalyzed
Enantioselective Allylic Substitutions with Readily Accessible Aryl- or
Heteroaryllithium Reagents and Aluminum Chlorides**
Fang Gao, Yunmi Lee, Kyoko Mandai, and Amir H. Hoveyda*
Catalytic enantioselective allylic substitution (EAS) reac-
tions[1] are among the most versatile classes of transforma-
tions in organic chemistry: such processes convert readily
available achiral substrates to enantiomerically enriched
products bearing a stereogenic center adjacent to a readily
functionalizable alkene. We have devised chiral amino acid-
based[2] and bidentate N-heterocyclic carbene (NHC) Cu
complexes[3] that promote EAS processes with dialkylzinc
reagents and generate quaternary carbon stereogenic cen-
ters[4] with high site- and enantioselectivity. Catalytic enan-
tioselective Cu-free additions of alkylmagnesium halides to
allylic halides,[5] as well as dialkylzinc and trialkylaluminum
reagents[6] to phosphates, have also been developed. None-
Scheme 1. Practical, site- and enantioselective synthesis of versatile
theless, significant and compelling problems remain unad-
small molecules with an allylic all-carbon quaternary stereogenic center
by Cu-catalyzed allylic substitutions through the use of monoaryl- or
dressed. Catalytic EAS transformations involving aryl nucle-
ophiles and which deliver quaternary carbon centers are
notoriously scarce, and protocols that involve heteroarylme-
monoheteroaryl–metal reagents; LG=leaving group.
À
tals do not exist (including those that furnish tertiary C C
bonds). The only reported cases of aryl additions by catalytic
EAS that generate quaternary carbons correspond to Si-
substituted alkenes and involve diarylzinc reagents,[7] which
are relatively difficult to prepare in high purity and offer only
one of the two aryl units. Such considerations underline a
shortcoming in the state-of-the-art: the absence of EAS
methods that deliver quaternary carbon stereogenic centers
through reactions with monoaryl–metal reagents, in particular
those that are easily accessible and atom-economical.[1]
Herein, we introduce the first set of efficient catalytic
EAS processes that involve the use of aryllithium or hetero-
aryllithium reagents and generate quaternary carbon stereo-
genic centers (Scheme 1). Reactions proceed with exceptional
site- (up to > 98% SN2’) and high enantioselectivity (up to
> 98:2 e.r.); transformations are complete within three hours
with ꢀ 3.0 mol% of a chiral NHC–Cu complex derived from
commercially available and air-stable CuCl2·2H2O.
We began by developing a protocol that only requires
aryllithium reagents, and decided to take advantage of the
higher propensity of an aryl unit of an (aryl)dialkylaluminum
to transfer to copper;[8] reaction of an aryllithium with a
commercially available and inexpensive dialkylaluminum
halide would deliver the desired reagent.[9] Thus, EAS with
phenyl(diethyl)aluminum, synthesized and used in situ from
phenyllithium, and allylic phosphate 5 was investigated
(Table 1). The ability of four representative chiral NHC–Cu
complexes, obtained from bidentate 1 and 2a[10] and mono-
dentate 3 and 4,[11] to promote the C C bond-forming
À
processes was examined. Initial studies indicated that,
although all reactions proceed efficiently (Ph/Et transfer
> 98:2), the Cu complex derived from sulfonate-bearing 2a
furnishes the highest site- and enantioselectivity.[12] It should
be noted that PhMgCl can also be used to access 6 with
similar efficiency and selectivity (> 98% conv., > 98% SN2’,
89.5:10.5 e.r.).[13] Reaction of the Grignard reagent with
Et2AlCl is performed in dioxane so that residual magnesium
halide (MgCl2·dioxane) can be removed.[14]
[*] F. Gao, Y. Lee, K. Mandai, Prof. A. H. Hoveyda
Department of Chemistry, Merkert Chemistry Center
Boston College
An assortment of aryllithium reagents, either purchased
or prepared by well-established procedures through metal–
halogen exchange with the corresponding bromides, can be
used in site-selective (> 98% SN2’), efficient (81–98% yield)
and enantioselective reactions (Table 2).[15] Cu-catalyzed
additions with a,b-unsaturated esters (entries 1–4, Table 2)
afford the EAS product in up to 91.5:8.5 e.r.; reactions with
Si-substituted allylic phosphates deliver the desired allylsi-
lanes in up to 97:3 e.r. (entries 5 and 6, Table 2). All trans-
formations proceed with > 98% SN2’ selectivity. The reactions
Chestnut Hill, MA 02467 (USA)
Fax: (+1)617-552-1442
E-mail: amir.hoveyda@bc.edu
[**] The NIH (GM-47480) provided financial support. Y.L. is grateful for
an AstraZeneca graduate fellowship. Mass spectrometry facilities at
Boston College are supported by the NSF (CHE-0619576).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201005124.
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Table 1: Initial examination of various chiral NHC complexes.[a]
reactions of the related aryl-substituted substrates with
alkylmetal reagents, such processes remain largely unex-
plored (the only reported variant involves dialkylzinc
reagents[2d]). Regardless of impending future developments,
diaryl-substituted products, such as 8 [Eq. (1)], can be most
Entry
NHC–AgI
Ph/Et
SN2’/SN2[b]
e.r.[c]
addition[b]
1
2
3
4
1
2a
3
>98:2
>98:2
>98:2
>98:2
>98:2
>98:2
56:44
83:17
86:14
91:9
89:11
58:42
4
[a] Reactions were performed under N2 atmosphere; >98% conversion
1
in all cases. [b] Determined by analysis of 400 MHz H NMR spectra of
unpurified mixtures. [c] Determined by GLC analysis; see the Supporting
Information for details.
easily accessed by an effective catalytic arylmetal addition.
Another notable feature of the Cu-catalyzed EAS in Equa-
tion (1) is that a higher enantiomeric purity is obtained with
modified complex 2b; when 2a is used in the latter instance,
26% ethyl addition product is observed (vs. < 2% with 2b)
and 8 is formed in 90:10 e.r. Similarly, the EAS in entry 4 of
Table 2 requires complex 1, since with 2a, the desired product
is isolated in 78.5:21.5 e.r. (vs. 83:17 e.r.).
Next, we probed the possibility of catalytic enantioselec-
tive additions of heterocyclic units. Efficient and site-selective
deprotonation of furan (Scheme 2) can be followed by
subjection of the resulting 2-(furyl)lithium to Et2AlCl to
afford (2-furyl)Al(Et)2. The resulting heteroarylaluminum
can be utilized without isolation or purification in a highly
group- (< 2% Et addition), site- (> 98% SN2’) and enantio-
selective (> 98:2 e.r.) process, converting allylic phosphate 5
to a-furyl ester 9 within 1 hour with 0.5 mol% 2c. When 2a is
utilized in this case, 9 is obtained with equally high efficiency
and site-selectivity (> 98% conv., > 98% SN2’, > 98% furyl
in entries 5 and 6, affording Si-substituted quaternary car-
bons, are substantially more efficient than those previously
reported involving diarylzincs.[3c] For example, with
(pOMeC6H4)2Zn, 2.5 mol% 2a and 24 h of reaction time
are needed (vs. 1.0 mol% and 3 h needed in entry 6), and the
desired product is isolated in 88:12 e.r. (vs. 97:3 e.r. with the
arylaluminum reagent). Furthermore, the (CuOTf)2·C6H6 salt
required for reactions with diarylzincs must be handled under
rigorously inert conditions as opposed to the air-stable
CuCl2·2H2O utilized here.
Although the products obtained through the transforma-
tions shown in Table 2 can, in principle, be accessed by
Table 2: Synthesis and in situ use of arylaluminum reagents in Cu-catalyzed enantioselective allylic substitution reactions.[a]
Entry
Aryllithium
Substrate
(R; R1)
NHC–AgI
[mol%]
t [h]
SN2’/SN2[b]
Yield [%][c]
e.r.[d]
1
2
3
4
5
6
PhLi
PhLi
pOMeC6H4Li
pCF3C6H4Li
PhLi
CO2tBu; Me
CO2tBu; Et
CO2tBu; Me
CO2tBu; Me
SiMe2Ph; Me
SiMe2Ph; Me
2a; 0.5
2a; 0.5
2a; 0.5
1; 0.75
2a; 1.0
2a; 1.0
0.5
3.0
1.0
1.5
3.0
3.0
>98:2
>98:2
>98:2
>98:2
>98:2
>98:2
98
96
87
88
81
97
91:9
91.5:8.5
90.5:9.5
83:17
96:4
pOMeC6H4Li
97:3
[a] Reactions were performed under N2 atmosphere; >98% conversion in all cases. [b] Determined through analysis of 400 MHz 1H NMR spectra of
unpurified mixtures. [c] Yields of isolated purified products. [d] Determined by GLC or HPLC analysis; see the Supporting Information for details.
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Communications
The final stage involved reactions of
allylic phosphates with two alkyl sub-
stituents, a particularly challenging class
of substrates vis-ꢀ-vis obtaining excep-
tionally
high
enantioselectivities
(Scheme 3). Transformations proceed
readily to afford the EAS products in
88–97% yield and with > 98% site
selectivity. Reactions with a methyl and
an n-alkyl group (cf. sporochnol, 11, and
12, 78.5:21.5–91:9 e.r.) are typically less
enantioselective than those bearing a
larger cyclohexyl unit (cf. 13–16, 91:9–
98:2 e.r.). The enantiotopic faces of the
alkenes in substrates in Scheme 3 are
difficult to differentiate due to similar
Scheme 2. Synthesis and in situ utilization of (2-furyl)AlEt2 in a highly group-, site- and
enantioselective NHC–Cu-catalyzed allylic substitution reaction, affording an all-carbon quater-
nary stereogenic center.
addition) but with diminished enantiomeric purity (90.5:9.5
vs. > 98:2 e.r.).
size and nearly identical electronic attributes of their alkyl
substituents. Products are therefore obtained with lower
enantioselectivities compared to aryl-, silyl-, and carboxylic
ester-substituted alkenes in Scheme 2 or Table 3.
The data in Table 3 illustrate that furyl- and thienyl-
(diethyl)aluminum[16] participate in efficient (> 98% conv. in
1.0 h with 0.5–1.5 mol% chiral NHC–AgI complex) reactions
that afford products with high enantioselectivity (86.5:13.5 to
> 98:2 e.r.) and yield (86–98% after purification). Substrates
containing an aryl substituent or those that bear a carboxylic
ester (entry 11, Table 3) or a silyl unit (entries 6 and 14,
Table 3) undergo notably facile and selective transformations.
Reactions with 3-furyl- and 3-thienyl(diethyl)aluminum begin
with readily available (bromo)heterocyclic precursors, which
are converted to the derived (heteroaryl)aluminum reagents
through a simple metal–halogen exchange procedure.[17]
The transformation with allylic phosphate 10 and the
aryl(diethyl)aluminum bearing a p-methoxyaryl unit affords
R-(À)-sporochnol. The same natural product was previously
synthesized by a process involving an aryl-substituted allylic
phosphate with di(2-methyl-2-pentenyl)Zn, promoted by
10 mol% of an amino acid-based chiral Cu complex.[2a]
Although the earlier version is more enantioselective
(91:9 e.r. vs. 78.5:21.5 e.r.), the EAS depicted in Scheme 3 is
superior on several fronts. First, 1.0 mol% of the NHC–Cu
complex is required for a reaction that is complete in 1 h at
À308C versus 10 mol% of the amino acid–Cu complex for
48 h at À788C (lower temperature required for maximum
enantioselectivity). Second, the combination of an aryllithium
and commercially available Et2AlCl, which is used in situ, is
considerably more practical than the preparation, purification
and use of the aforementioned dialkylzinc reagent. Third,
catalytic EAS with an arylmetal reagent requires allylic
phosphate 10, which is easily accessed in one step from
geraniol (vs. Horner–Emmons olefination and reduction of
ester, followed by phosphate formation).
Table 3: Synthesis and in situ use of heterocyclic aluminum reagents in
Cu-catalyzed enantioselective allylic substitution reactions.[a]
In brief, we address a longstanding shortcoming in an
À
important class of enantioselective C C bond-forming reac-
Entry Heterocycle Substrate mol% SN2’/SN2[b] Yield e.r.[d]
tions. The present investigations introduce efficient and
highly selective Cu-catalyzed EAS reactions that provide
access to numerous versatile molecules, containing aryl- or
heteroaryl-substituted quaternary carbon stereogenic centers,
and which cannot be easily prepared by any alternative
protocols.[18] Moreover, the investigations outlined herein
offer additional evidence regarding the unique ability of
sulfonate-bridged bidentate NHC-based metal complexes to
provide an effective solution to difficult problems in catalysis.
(R)
2a
[%][c]
1
2
3
4
5
6
2-furyl
2-furyl
2-furyl
2-furyl
2-furyl
2-furyl
3-furyl
2-thienyl
2-thienyl
2-thienyl
2-thienyl
Ph
oBrC6H4
0.5
0.5
>98:2
>98:2
>98:2
>98:2
>98:2
>98:2
>98:2
>98:2
>98:2
>98:2
>98:2
>98:2
>98:2
>98:2
93
86
95
98
96
91
90
98
98
96
94
94
89
95
>98:2
>98:2
>98:2
98:2
>98:2
86.5:13.5
97:3
96:4
98:2
94:6
87.5:12.5
94:6
oMeOC6H4 0.5
oMeC6H4 1.0
pNO2C6H4 0.5
SiMe2Ph
Ph
Ph
oBrC6H4
pNO2C6H4 0.5
CO2tBu
Ph
1.0
1.0
0.5
1.5
7[e]
8
9
10
11
Received: August 16, 2010
Published online: September 30, 2010
0.5
1.0
1.0
1.0
12[e] 3-thienyl
13[e] 3-thienyl
14[e] 3-thienyl
oBrC6H4
SiMe2Ph
97:3
94:6
Keywords: allylic substitution · aluminum reagents ·
enantioselective reactions · N-heterocyclic carbenes ·
quaternary carbon centers
[a–d] See Table 2. [e] The corresponding 3-bromofuran or 3-bromothio-
phene were used as starting materials (treatment with nBuLi in Et2O);
see the Supporting Information for experimental details.
.
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[6] For Cu-free allylic alkylation reactions
promoted by NHC–ZnII and NHC–AlIII
complexes, see: Y. Lee, B. Li, A. H. Hov-
eyda, J. Am. Chem. Soc. 2009, 131, 11625 –
11633.
[7] a) Ref. [3c]. For catalytic EAS reactions
that involve arylmetals but deliver tertiary
À
C C bonds, see: b) K. B. Selim, K-i.
Yamada, K. Tomioka, Chem. Commun.
2008, 5140 – 5142; c) K. B. Selim, Y. Matsu-
moto, K.-I. Yamada, K. Tomioka, Angew.
Chem. 2009, 121, 8889 – 8891; Angew.
Chem. Int. Ed. 2009, 48, 8733 – 8735;
d) C. A. Falciola, A. Alexakis, Chem. Eur.
J. 2008, 14, 10615 – 10627; e) D. Polet, X.
Rathgeb, C. A. Falciola, J. B. Langlois, S. E.
Hajjaji, A. Alexakis, Chem. Eur. J. 2009, 15,
1205 – 1206.
[8] For the more favored transfer of an aryl unit
versus an alkyl group of an Al-based
reagent, see: a) T. Mole, J. R. Surtees,
Aust. J. Chem. 1964, 17, 310 – 314; b) N. A.
Bumagin, A. B. Ponomaryov, I. P. Belet-
skaya, Tetrahedron Lett. 1985, 26, 4819 –
4822; c) B. Z. Lu, F. Jin, Y. Zhang, X. Wu,
S. A. Wald, C. H. Senanayake, Org. Lett.
2005, 7, 1465 – 1468; d) E. Merino, R. P. A.
Melo, M. Ortega-Guerra, M. Ribagorda,
M. C. Carreꢃo, J. Org. Chem. 2009, 74,
2824 – 2831; e) H. Gao, P. Knochel, Synlett
2009, 1321 – 1325.
Scheme 3. NHC–Cu-catalyzed enantioselective additions of aryl and heteroaryl moieties to
substrates that bear two alkyl substituents. All reactions performed under the conditions
shown for EAS reaction leading to sporochnol, except for 15 and 16. [a] With 1.0 mol%
NHC–AgI 2a and 2.0 mol% CuCl2·2H2O. [b] With 1.0 mol% NHC–AgI 2a and 2.0 mol%
CuCl2·2H2O, À508C, 3 h.
[9] For use of this procedure in Cu-catalyzed
enantioselective conjugate additions, see:
a) T. L. May, M. K. Brown, A. H. Hoveyda,
Angew. Chem. 2008, 120, 7468 – 7472;
[1] For reviews on catalytic enantioselective allylic substitution
Angew. Chem. Int. Ed. 2008, 47, 7358 – 7362. For a related
study involving a relatively limited substrate range and Feringa-
type phosphoramidite ligands, see: b) C. Hawner, K. Li, V.
Cirriez, A. Alexakis, Angew. Chem. 2008, 120, 8334 – 8337;
Angew. Chem. Int. Ed. 2008, 47, 8211 – 8214. Conjugate additions
with heteroarylaluminum reagents have not been reported.
[10] For reports on the utility of sulfonate-based bidentate NHC–Cu
complexes in enantioselective formation of quaternary carbon
stereogenic centers, see: a) M. K. Brown, T. L. May, C. A.
Baxter, A. H. Hoveyda, Angew. Chem. 2007, 119, 1115 – 1118;
Angew. Chem. Int. Ed. 2007, 46, 1097 – 1100; b) Ref. [3c];
c) Ref. [9a]; d) M. K. Brown, A. H. Hoveyda, J. Am. Chem.
Soc. 2008, 130, 12904 – 12906; e) Ref. [6]; f) A. Guzman-Marti-
nez, A. H. Hoveyda, J. Am. Chem. Soc. 2010, 132, 10634 – 10637;
g) F. Gao, K. P. McGrath, Y. Lee, A. H. Hoveyda, J. Am. Chem.
Soc. 2010, 132, ASAP.
(EAS) reactions with “hard” C-based nucleophiles, see: a) A. H.
Hoveyda, A. W. Hird, M. A. Kacprzynski, Chem. Commun.
2004, 1779 – 1785; b) H. Yorimitsu, K. Oshima, Angew. Chem.
2005, 117, 4509 – 4513; Angew. Chem. Int. Ed. 2005, 44, 4435 –
4439; c) A. Alexakis, J. E. Bꢁckvall, N. Krause, O. Pꢀmies, M.
Diꢂguez, Chem. Rev. 2008, 108, 2796 – 2823.
[2] a) C. A. Luchaco-Cullis, H. Mizutani, K. E. Murphy, A. H.
Hoveyda, Angew. Chem. 2001, 113, 1504 – 1508; Angew. Chem.
Int. Ed. 2001, 40, 1456 – 1460; b) K. E. Murphy, A. H. Hoveyda,
J. Am. Chem. Soc. 2003, 125, 4690 – 4691; c) M. A. Kacprzynski,
A. H. Hoveyda, J. Am. Chem. Soc. 2004, 126, 10676 – 10681;
d) K. E. Murphy, A. H. Hoveyda, Org. Lett. 2005, 7, 1255 – 1258.
[3] a) A. O. Larsen, W. Leu, C. N. Oberhuber, J. E. Campbell, A. H.
Hoveyda, J. Am. Chem. Soc. 2004, 126, 11130 – 11131; b) J. J.
Van Veldhuizen, J. E. Campbell, R. E. Giudici, A. H. Hoveyda,
J. Am. Chem. Soc. 2005, 127, 6877 – 6882; c) M. A. Kacprzynski,
T. L. May, S. A. Kazane, A. H. Hoveyda, Angew. Chem. 2007,
119, 4638 – 4642; Angew. Chem. Int. Ed. 2007, 46, 4554 – 4558.
[4] a) Quaternary Stereocenters: Challenges and Solutions for
Organic Synthesis (Eds.: J. Christophers, A. Baro), Wiley-
VCH, Weinheim, 2006; b) P. G. Cozzi, R. Hilgraf, N. Zimmer-
mann, Eur. J. Org. Chem. 2007, 5969 – 5994.
[5] For Cu-free allylic alkylations with alkylmagnesium halides,
affording all-carbon quaternary stereogenic centers, see: a) Y.
Lee, A. H. Hoveyda, J. Am. Chem. Soc. 2006, 128, 15604 – 15605.
For a closely related study with derivatives of bidentate NHC
complexes originally designed in these laboratories, see: b) O.
Jackowski, A. Alexakis, Angew. Chem. 2010, 122, 3418 – 3422;
Angew. Chem. Int. Ed. 2010, 49, 3346 – 3350.
[11] For C1-symmetric chiral NHC–Cu complexes and their utility in
À
À
À
conjugate additions resulting in C C, C Si or C B bonds,
respectively, see: a) K-s. Lee, A. H. Hoveyda, J. Org. Chem.
2009, 74, 4455 – 4462; b) K.-s. Lee, A. H. Hoveyda, J. Am. Chem.
Soc. 2010, 132, 2898 – 2900; c) J. M. OꢄBrien, K-s. Lee, A. H.
Hoveyda, J. Am. Chem. Soc. 2010, 132, 10630 – 10633.
[12] When Me2AlCl or (iBu)2AlCl are used, alkyl (vs. aryl) transfer
products are generated (12% and 10%, respectively) and 6 is
formed in 73:27 and 88:12 e.r., respectively.
[13] When PhMgBr is used, site- and enantioselectivity remain the
same but efficiency is diminished (68% vs. > 98% conv. with
PhMgCl).
[14] Metal halides can be detrimental to enantioselectivity. For
example, when the reaction in entry 2 of Table 1 is performed
with an added equivalent of LiCl, the desired product is obtained
in ca. 70:30 e.r. Arylaluminum solutions are allowed to stand for
Angew. Chem. Int. Ed. 2010, 49, 8370 –8374
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Communications
30 min to 1 h to permit precipitation of LiCl to occur prior to the
supernatant being utilized. See the Supporting Information for
experimental details.
[15] Reactions with ortho-substituted arylaluminums are efficient
and site-selective (> 98% SN2’) but furnish products of lower
enantiomeric purity. For example, when (diethyl)o-methylphe-
nylaluminum is used with the allylic phosphate in entries 1–4 of
Table 2, the EAS product is obtained in 69.5:30.5 e. r. In the
corresponding conjugate addition processes, such arylaluminums
give rise to improved e.r. values (see Ref. [9a]).
[16] For use of furyl- and thienylaluminum reagents in enantioselec-
tive additions to carbonyls, see: a) K-H. Wu, D.-W. Chuang, C.-
A. Chen, H.-M. Gau, Chem. Commun. 2008, 2343 – 2345;
b) D. B. Biradar, S. Zhou, H.-M. Gau, Org. Lett. 2009, 11,
3386 – 3389.
[17] As the example below indicates, EAS with N-containing
heterocyclic substrates proceed readily but are hampered by
non-selective initial heteroaryllithium formation, affording a
difficult to separate mixture of products. See the Supporting
Information for details. Use of the pyridine-containing alumi-
num reagents leads to < 2% conversion. Studies to improve the
above transformations are in progress.
[18] Ni-catalyzed reactions of styrenes with ethylene furnish enan-
tiomerically enriched products with a benzylic vinyl unit at an
all-carbon quaternary stereogenic center. See: C. R. Smith, H. J.
Lim, A. Zhang, T. V. RajanBabu, Synthesis 2009, 2089 – 2100,
and references therein.
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